Gas-liquid absorption method is still a powerful tool for separating and purifying gas. Absorbents commonly used in the liquid solution for separating carbon dioxide include alkanolamines, alkaline salt and their modified forms. To improve the performance, a chemical compound may be added to the liquid phase, which can react with the gas to form other compounds. A chemical reaction may have two beneficial effects on the absorption process. First, it can increase the carrying capacity of the liquid solution. Second, it can reduce mass transfer resistance or increase mass transfer coefficient. Both effects contribute to an increased absorption rate. However, such chemical reactions may hinder the release of the absorbed gas from the liquid solution.
R. H. Niswander, D. J. Edwards, M. S. DuPart, and. J. P. Tse, in an article entitled “A More Energy Efficient Product for Carbon Dioxide Separation”, Separation Science and Technology. 28, no. 1–3, (January 1993) pp. 565–578 suggested that aqueous solution of alkanolamines was widely used to separate carbon dioxide from other gases to meet the requirement of very low residual carbon dioxide concentration. Amine compounds such as monoethanolamine (MEA) and diethanolamine (DEA) can undergo side reactions with carbon dioxide to form a variety of degradation compounds. These compounds reduce the performance of the solvent, cause corrosion on the reaction vessel, and increase energy consumption of the system.
Another popular method for separating carbon dioxide employs an alkaline salt solution as an absorbent. Sodium carbonate and potassium carbonate are the most commonly used materials. Absorption process can be divided into two types based on temperature at which the process is carried out: ambient temperature (70–100° C.) and elevated temperature (approximating regeneration temperature). Slow absorption rates at an ambient temperature result in a low efficiency of carbon dioxide recovery. Therefore, a large amount of steam is necessary to regenerate the absorbent. In comparison, absorption at an elevated temperature such as in Benfield Process overcomes some of the disadvantages observed in the ambient-temperature process. By increasing the temperature of the absorption process, both the absorption rate and the absorbent's gas-holding capacity increase.
Several modifications have been adopted in some processes to accelerate the absorption rates of carbon dioxide. In those processes, activators or promoters are added into carbonate solutions. Those processes include Benfield Process, the Cartacarb Process, and the Glammarco-Vetrocoke process. Arthur L. Kohl and Fred C. Riesenfeld in their book “Gas Purification”, Gulf Publishing Company (1985) at page 235, discuss the effects of promoters and activators on the carbon dioxide absorption rate and vapor-liquid equilibria. Compared with hot potassium carbonate solutions, diethanolamine (DEA) and sterically hindered amines were found to be very effective in increasing the absorption rate of carbon dioxide. However, the partial pressure of carbon dioxide at equilibrium decreases after an activator is added into the carbonated solution. This means that it is more difficult to recover carbon dioxide from the activated solution than from a solution containing no activator.
It is therefore apparent there is a need for a method that exhibits a high absorption rate without increasing the difficulty in regenerating the absorbent.